1. Field of the Invention
Until fairly recently, the technology pertaining to rubber-like materials (elastomers) as force-reducing impact media has not been well quantified because of the scarcity of actual use requirements and lack of suitable test equipment. However, automotive industry trends toward safety impact bumpers brought about by legislation have significantly increased research activities to acquire such technology. Such research activities should take into account the following theoretical considerations. Yielding materials on impact reduce impact forces by decelerating the impacting object through a distance. The maximum force generated is the product of the mass of the impacting object and its maximum deceleration. Thus, if an object decelerates through a short distance, forces are higher than if an object decelerates less rapidly through a longer distance. The advantage of using rubber-like materials as impact media is that the original shape is recoverable after the impact cycle.
Rubber-like materials must meet several criteria to serve effectively as force-reducing impact media (bumpers) for automobiles. The materials must provide sufficient resistance to the impacting object to stop the object in a reasonable distance so that acceptable forces on the automobile are obtained. Then the media must return to substantially its original dimensions. These criteria must hold over the service temperature and impact speed of the operation. Further, rubbers are viscoelastic in nature (i.e., their physical properties vary as a function of temperature and strain rate). Therefore, in the evaluation of rubber-like materials as energy absorbing impact components (bumpers) for automobiles, these factors must also be taken into account. Additionally, the evaluation should simulate closely actual use conditions. Prior art bumpers employing rubber-like materials as energy absorbing media have not been entirely satisfactory.
2. Description of the Prior Art
By way of illustration, U.S. Pat. No. 3,493,257 describes a composite urethane-steel bumper stated to be capable of withstanding the impact of a 4000 pound automobile traveling at five miles per hour with a stationary object without damage to the bumper. A urethane thickness of 1.5 inches and the formulation used to produce the urethane are disclosed in the patent. Based on an analysis of that disclosure, there are shortcomings both in the urethane formulation and in the bumper design described in the patent. The approximate force generated from the given impact conditions can be calculated from the equation: EQU F = V.sup.2 .times.W.times.0.186.times.E.sub.f /(X .times. E (A) where
X = stroke distance, inches
F = force, lbs.
V = velocity, ft/sec.
W = weight, lbs.
E.sub.f =energy fraction absorbed of total energy
E = Cycle Efficiency fraction
Assuming the stroke is 2/3 the thickness (1.0 inch), E.sub.f is 0.3 and E is 0.5, then the force (F) calculated from equation (A) is 32,800 pounds at impact. This force is well above the force expected to be capable of deforming the steel reinforcing bar of the composite automotive bumper described in the patent.
Further, the following formulation (reaction mixture) is described in U.S. Pat. No. 3,493,257.
______________________________________ Parts Per Hundred Ingredient (by weight) ______________________________________ Organic Polyisocyanate 35.8 Polyol 100 Aromatic Diamine 26.4 H.sub.2 O 0.05 ______________________________________
This formulation contains a 1000 molecular weight polyoxypropylene polyol and a relatively large amount of the aromatic diamine is employed. The moduli of such urethanes produced from such formulations are relatively sensitive with respect to temperature changes. The urethane becomes hard at cold temperatures thereby decreasing the amount of deflection from impact as the temperature is reduced. This effect increases object deceleration which results in increased forces on the automobile. Thus, from both urethane formulation and design considerations, the bumper of U.S. Pat. No. 3,493,257 is not suitable to meet the applicable Federal Motor Vehicle Safety Standard (i.e., FMVSS 215) requirements for a safety bumper.
As a further illustration U.S. Pat. No. 3,514,144 describes a realistic concept for an energy absorbing urethane elastomer bumper. The statement is made "with a bumper of modest dimensions, the energy of impact for a heavy automobile traveling at 5 mph can be fully dissipated by this contruction". However, no test data is given nor any description of the urethane elastomer presented. A temperature insensitive and energy absorbing elastomeric material would be required to make the urethane bumper of the latter patent functional.
Additionally, U.S. Pat. No. 3,558,529 discloses the use of a mixture of incompatible polyols for making temperature-insensitive urethane polymers that are useful as crash padding, insulation etc. However, the products of the latter patent are not entirely satisfactory energy absorbing materials. Thus, the specific disclosure of this patent (particularly the Examples) relate only to polyols which impart to the products insufficient load bearing properties for many energy absorbing applications, especially for automobile bumpers.
Further, U.S. Pat. No. 3,580,869 discloses that urethane automobile bumpers can be produced from reaction mixtures containing a polyol, an aromatic polyamine having unhindered primary amino groups, an organic polyisocyanate, and an aromatic glycol. Such reaction mixtures are relatively difficult to process unless the aromatic amine and the organic polyisocyanate are prereacted to form a prepolymer. Moreover, urethane bumpers so produced have relatively poor load-bearing properties and it is believed they have been found suitable for use only as "cosmetic" bumpers (as distinguished from energy absorbing bumpers). Improved load bearing and better processability are achieved using the polymer polyol-hindered aromatic amineorganic polyisocyanate formulations of U.S. Pat. No. 3,586,649 to produce the urethane. However, such urethanes lack the desired degree of strain rate sensitivity.